In this paper, we introduce an analytical framework to compute the average rate of downlink heterogeneous cellular networks. The framework leverages recent application of stochastic geometry to other-cell interference modeling and analysis. The heterogeneous cellular network is modeled as the superposition of many tiers of Base Stations (BSs) having different transmit power, density, path-loss exponent, fading parameters and distribution, and unequal biasing for flexible tier association. A long-term averaged maximum biased-received-power tier association is considered. The positions of the BSs in each tier are modeled as points of an independent Poisson Point Process (PPP). Under these assumptions, we introduce a new analytical methodology to evaluate the average rate, which avoids the computation of the Coverage Probability (Pcov) and needs only the Moment Generating Function (MGF) of the aggregate interference at the probe mobile terminal. The distinguishable characteristic of our analytical methodology consists in providing a tractable and numerically efficient framework that is applicable to general fading distributions, including composite fading channels with small- and mid-scale fluctuations. In addition, our method can efficiently handle correlated Log-Normal shadowing with little increase of the computational complexity. The proposed MGF-based approach needs the computation of either a single or a two-fold numerical integral, thus reducing the complexity of Pcov-based frameworks, which require, for general fading distributions, the computation of a four-fold integral.

The standardization of 5G systems is entering in its critical phase, with 3GPP that will publish the PHY standard by June 2017. In order to meet the demanding 5G requirements both in terms of large throughput and global connectivity, Satellite Communications provide a valuable resource to extend and complement terrestrial networks. In this context, we consider a heterogeneous architecture in which a LEO mega-constellation satellite system provides backhaul connectivity to terrestrial 5G Relay Nodes, which create an on- ground 5G network. Since large delays and Doppler shifts related to satellite channels pose severe challenges to terrestrial-based systems, in this paper we assess their impact on the future 5G PHY and MAC layer procedures. In addition, solutions are proposed for Random Access, waveform numerology, and HARQ procedures.

Universal connectivity has been part of past and current generations of wireless systems, but as we approach the sixth generation (6G), the subject of social responsibility is being built as a core component. Given the advent of Non-Terrestrial Networks (NTN), reaching these goals will be much closer to realization than ever before. Owing to the benefits of NTN, the integration of NTN and Terrestrial Networks (TN) is still at its infancy, where the past, the current and future standard releases within the 3^rd Generation Partnership Project (3GPP) provide guidelines to adopt a successfully co-existence/integration of TN and NTN. Therefore, in this article, we illustrate, through 3GPP guidelines, how NTN and TN can effectively be integrated. Moreover, the role of beamforming and Artificial Intelligence (AI) is highlighted to achieve this integration. Finally, the usefulness of integrating NTN and TN is validated through numerical analysis.

In this paper we present the concept of cognitive satellite terrestrial radios (CSTR) for hybrid satellite-terrestrial systems (HSTS). The cognitive radio (CR) technology is being motivated by the radio regulatory bodies for efficient utilization of the radio spectrum. The future satellite ground terminals therefore need to integrate and co-exist with the spectrally crowded terrestrial wireless systems and hence we propose the idea of CSTR. The key issues are addressed and the concepts of developing CR based satellite ground terminals in HSTS for dynamic spectrum access on the ground are presented. Furthermore, the concept of 3D-Spatial reuse of the spectrum is also presented using the CSTR considering low elevated satellite ground stations. Two particular HSTS applications to illustrate the CSTR concept, a) the hybrid satellite-UWB (ultra wideband) communication system for personal area networks (PAN) with short range on ground communications, and b) the hybrid satellite-WRAN (wireless regional area networks) for long range on ground communications are presented. In both the applications the satellite uplink and the terrestrial radios adopt the CR functionalities. The key enabling technologies and their integrated architecture for the CSTR are also presented.

Today, the worldwide economy and society are already taking advantage of the first deployments of 5G networks. However, the challenges that will be posed by 2030 networks on the infrastructure, technologies, and performance requirements call for a further leap in mobile communications. To this aim, evolutions of the 5G concepts in 5G-Advanced, and rethinking of future Radio Access Technologies, i.e., 6G which is expected to be the next revolution in the mobile ecosystem, are already being addressed by the scientific and industrial communities. In this framework, as demonstrated by 3GPP Rel. 17 specifications, Non-Terrestrial Networks (NTN) are expected to play a vital role in near and long term mobile communications, based on the joint optimisation of the terrestrial and non-terrestrial components in a unified 3D architecture. In this paper, we extensively discuss the 3GPP normative activities for 5G (Rel. 17), 5G-Advanced (Rel. 18 onward), and 6G (Rel. 20+) in terms of enabling features, architectures and interfaces, and protocols. Then, we explore the evolution of the 5G services towards 5G-Advanced and 6G NTN applications. Finally, we discuss the evolution to 6G network infrastructures with respect to the main performance requirements.